Reported May 2005

Monsoon

WHAT ARE MONSOONS? Many people associate monsoons with heavy rain, but the term actually refers to a wind shift rather than precipitation -- although monsoons are often accompanied by rain and thunderstorms, usually from June through September.

Technically, monsoons are major wind patterns that reverse direction depending on the season. While monsoons are beneficial, even critical for survival in many regions -- they supply almost 90 percent of India's water supply, for instance -- they can also cause floods, drought, damage, deaths, and destroy crops.

Monsoons are caused by sharp contrasts in temperature between the land and the water. There are two main types of monsoon, characterized by the earth's tilt in comparison to the sun. Because the monsoon is so closely tied to the movement of the sun, it occurs fairly regularly every year.

Summer monsoons form when the earth is tilted and the sun's rays heat the air in the northern hemisphere, so that the land heats up rapidly, drawing humid air in from surrounding bodies of water, which rises and condenses into rain.

Winter monsoons occur when the sun's rays shine mostly on the southern hemisphere, and when the water is warmer than the land. Dry air flows from the land out over the ocean. So monsoon climates typically experience drought in winter and rain in summer.

Monsoons primarily occur in Asia and India, although smaller monsoons are found in equatorial Africa, northern Australia, and even the southwestern United States. For instance, during the winter, wind flow in Arizona moves from the west or northwest, shifting in the summer to a southerly or southeasterly direction. This pulls in moisture from the surrounding oceans, eventually triggering intense rains.

The American Meteorological Society contributed to the information in the TV portion of this report.

If you would like more information, please contact:

David Gochis, Ph.D.
NCAR Research Applications Program
Boulder, Colorado
(303) 497-2809

gochis@ucar.edu

NCAR Media Relations
(303) 497-8604

anatta@ucar.edu

During monsoon season in the Southwest, downdrafts in a thunderstorm can form dust walls on the ground, called haboobs. The name comes from the Arabic word "habb," meaning "wind." Wind speeds can top 30 MPH and the dust may rise to more than 3000 feet.

Did you know...

"Monsoon" comes from the Arabic word "mausam," meaning "season." Arab mariners coined the term to describe the seasonally shifting winds in the Indian Ocean and Arabian Sea.